In many constant speed drive systems, the need exists for an overpressure relief valve capable of discharging relief valve flows to multiple outlets depending upon pressure build-up rates. In particular, relief valves for oil systems are required between the unit scavenge pump and scavenge filter and also between the scavenge filter and the oil out boss. For instance, overpressure relief valves are commonly required for constant speed drive systems of the types utilized for a variety of applications such as in aircraft.
With regard to an overpressure relief valve utilized between the unit scavenge pump and the scavenge filter, such a valve relieves overpressure as a result of excessive flow resistance in the scavenge filter and/or downstream of the scavenge filter. The overpressure relief valve located between the scavenge filter and the oil out boss, on the other hand, typically relieves overpressure as a result of excessive flow resistance in the external cooling system. While a number of factors can produce an excessive flow resistance, the two most common are a reduced flow area due to filter plugging or high oil viscosity due to cold oil.
Generally, it is recognized that the oil relieved during these two conditions would provide more desirable performance if vented to two different locations in the system. For instance, during cold start tests, it has been determined that directing the filter overpressure relief valve overflow back to the inlet of the unit scavenge pump provides improved startability because this flow helps to decongeal and self-prime the scavenge pump. However, it has been suggested that relieving this overflow to the inlet of the unit scavenge pump during filter plugging sitautions may cause localized overheating, loss of charge pressure, and subsequent unit damage.
For plugged filter situations, it could be desirable to direct this overflow to a sump where temperature detectors could cause a shutdown of the constant speed drive system or to the inlet of the constant speed drive system to eliminate a loss of charge pressure. The specific routing for different applications will depend on individual system design. In any event, it is generally recognized that a multiple port relief valve is required which allows discharge of relief valve overflows to more than one location based on the pressure build-up rate and/or temperature of the oil and environment.
Among the attempts to provide a relief valve is that disclosed in Chichester U.S. Pat. No. 3,455,322. The relief valve disclosed therein is a pressure compensated diverter valve for lift systems with hydraulic pumps to relieve system pressure during pressure surges wherein there is always flow through a fluid outlet passage. Blake U.S. Pat. No. 4,294,278 discloses a pressure modulating valve whereby a fluid supplied to clutches is modulated from a low pressure to a high pressure to provide smooth clutch engagement. The modulating valve includes a discharging port for supplying a fluid under pressure to clutches and a pair of discharging ports in communication with a sump. Among other attempts to provide valves for various applications are those disclosed in U.S. Pat. Nos. 2,630,294; 2,737,196; 2,818,711; 2,898,934; 3,185,138; 3,419,030; 3,500,854; 3,687,154; 3,749,121; 3,916,946; 3,967,644 and 4,278,928.
In practice, it has remained to provide a multiple drain relief valve which is pressure responsive in a highly effective manner so as to control communication between an inlet and a given outlet as a function of the prevailing fluid pressure. While accomplishing such objectives, it is desirable to provide a relief valve which functions such that there is no flow at certain pressures, flow to a first outlet as a result of a rapid pressure increase, and flow to a second outlet as a result of a gradual pressure increase. Further, it is desirable to provide a relief valve for utilization in a constant speed drive system of the type found in a variety of applications including those commonly provided for aircraft.
The present invention is directed to overcoming the above stated problems and accomplishing the stated objects.